1. Field of the Invention
The present invention relates to a method for improving the accuracy of non-destructive and non-intrusive testing techniques of moving web-like materials, such as paper, paperboard and other porous materials produced by the paper industry and, in particular, a system for correcting certain inaccuracies generally inherent with ultrasonic testing systems.
2. Description of the Prior Art
Web-like materials, such as paper, paperboard and the like are required to meet particular mechanical property specifications. Normal quality control techniques require that the web-like materials be tested to ensure that the web uniformly meets the desired mechanical property specification.
Destructive-type tests are known for measuring mechanical properties of such web-like materials. Such destructive tests are normally conducted off line on representative samples of the web. There are various problems with such off-line destructive testing. For example, such testing is relatively time consuming and requires production to be stopped or sampled periodically when product is received from the machine. In addition, since such testing is destructive, it is normally performed on representative samples of the web which may be taken, for example, every several thousand of square feet of material. In such a situation, a substantial amount of waste is incurred if the web-like material is found to fail the test.
In order to solve the problems associated with such destructive test type measurements of mechanical properties of web-like materials, ultrasonic testing techniques have been developed. Such testing is done on-line and thus is relatively quicker than off-line destructive testing, while at the sane time provides relatively continuous indication of various mechanical properties of the web-like material to assure virtually uniform quality of the product while minimizing waste.
In known ultrasonic systems for testing various mechanical properties, two ultrasonic transducers are provided. The ultrasonic transducers are generally disposed on opposing sides of the web to allow ultrasonic signals to be transmitted in a direction generally normal to the plane of the web. In such a system, one transducer acts as a transmitter while the other transducer as a receiver. The time of flight of an ultrasonic signal through the thickness of the web is sensed to determine various mechanical properties of the web. In particular, the time of flight of the ultrasonic signal through the thickness of the web is determined by measuring the time of flight of the ultrasonic signal between the transducers during a condition when there is no web or sample present and then measuring the time of flight of the ultrasonic signal during a condition when a web is disposed between the transducers. Generally speaking, the difference in the times of flight during the two conditions is representative of the time of flight of the ultrasonic signal through the web.
There are several known problems with such known ultrasonic testing systems. More specifically, such ultrasonic testing systems normally utilize a digitizing oscilloscope for converting the ultrasonic signals to digital values in order to enable the time of flight to be computed by a digital computer. Such digitizing oscilloscopes are subject to errors from a phenomenon known as "trigger jitter". Trigger jitter in general relates to the unpredictability in the difference in time between the receipt of a trigger signal at the digitizing oscilloscope and the onset of the analog-to-digital conversion. Since this time period can vary with each sample period, cross-correlation of time of flight data with and without a sample present can result in certain inaccuracies in the time of flight measurements and, in turn, the mechanical property being tested.
Another source of error in the time of flight measurement in such ultrasonic testing systems relates to variances in the acoustic path between the two transducers. More particularly, such ultrasonic transducers, normally formed from ceramic or plastic piezoelectric material, are disposed in fluid-filled wheels in order to provide acoustical coupling of the ultrasonic signals relative to the web-like material. Such transducers are fixed in place and attached to axes about which the wheels rotate. Variances in the thickness of the wheels about their circumference can cause errors in the flight time measurements resulting in inaccurate mechanical property measurements.